The long-term objective of this proposal is to understand the molecular mechanism by which cAMP modulates differentiation of the dimorphic, pathogenic fungus, Histoplasma capsulatum. H. capsulatum is the causative agent of the disease, histoplasmosis, one of the most common primary systemic fungal infections in the Western Hemisphere. Millions of people throughout the world have become infected with the organism, and the number of reported cases of disseminated histoplasmosis in patients with AIDS is increasing at an alarming rate. The organism exists in soil as multicellular, filamentous mycelia, but the only form of the fungus found in infected tissue is unicellular, budding yeast. Since the mycelial phase of H. capsulatum is nonpathogenic and only the yeast phase is parasitic, factors which control the phase transition are critical for understanding pathogenicity. H. capsulatum is easily maintained in the laboratory by controlling the temperature of incubation; yeast grow at 37 degrees C and mycelia at 25 degrees C. By shifting the temperature of incubation, the transition from one phase to the other is induced. One of the most promising candidates for the transducing signal that responds to temperature and triggers morphogenesis, is cAMP. Exposure of yeast cultures to agents which raise the intracellular level of cAMP, induces the transition to mycelia at the nonpermissive temperature of 37 degrees C. The specific aim of this proposal is to determine whether ras function is involved in the cellular response to cAMP in H. capsulatum. In yeast, RAS proteins control GTP-stimulated adenylate cyclase activity and at least one functional RAS gene is essential for cell viability, proliferation and spore germination. To determine whether ras function is involved in the differentiation of H. capsulatum, the RAS genes will be cloned and characterized by standard molecular biological techniques. Expression of these genes during morphogenesis will be assessed by Northern analysis, and biochemical methods will be utilized to determine whether RAS genes in H. capsulatum function in the cAMP effector pathway.